The present invention relates to a process for purifying gallium alkoxides useful for the formation of dielectrics containing gallium oxide and the syntheses of compounds for photoelectronics.
The gallium alkoxides have been studied as raw materials for the formation of the dielectrics containing gallium oxide such as NdGaO3 and LaSrGaO4 and raw materials for the syntheses of the compounds for photoelectronics.
For the preparation of the gallium alkoxides, R. C. Mehrotra and R. K. Mehrotra, Current Sci. (India), Vol. 33 (8), 241 (1964) disclose a process for preparing gallium isopropoxide by reaction of sodium isopropoxide with gallium chloride in benzene solvent. The reaction of the gallium isopropoxide with other alcohols to allow the preparation of gallium methoxide, gallium ethoxide, gallium n-propoxide, gallium butoxide, and the like is disclosed by S. R. Bindal, V. K. Mathur, and R. C. Mehrotra, J. Chem. Soc., A863 (1969).
As described above, in general, gallium isopropoxide (hereinafter represented by Ga(OiPr)3) important as a starting material for the gallium alkoxides is prepared by the reaction of sodium isopropoxide (hereinafter represented by NaOiPr) with gallium chloride (hereinafter represented by GaCl3).
However, there has been no known literature on the content of chlorine contained as an impurity in Ga(OiPr)3 prepared by this process. The present inventors carried out the reaction of GaCl3 with NaOiPr equivalent or more thereto, subsequently the recovery of the resulting Ga(OiPr)3 by distillation, and the analysis of a chlorine content, and as a result found that the product is contaminated by approximately 1 weight percent of chlorine.
Parts for electronic elements contaminated by a chlorine impurity cause deterioration in the performances or lives thereof. Therefore, reduction in the chlorine content of the gallium alkoxides has been demanded. In particular, reduction in the chlorine content of Ga(OiPr)3 that is a raw material for various compounds has been demanded.
The invention aims at providing a process for purifying the gallium alkoxides to reduce the content of a chlorine impurity.
The invention provides the process for purifying the gallium alkoxides characterized by adding potassium alkoxides to the gallium alkoxides containing chlorine compounds as impurities to allow both alkoxides to react in an organic solvent and subsequently recovering the gallium alkoxides by distillation or sublimation.
Furthermore, the invention provides the process for purifying gallium isopropoxide characterized by adding potassium isopropoxide to gallium isopropoxide containing chlorine compounds as impurities to allow both isopropoxides to react in an organic solvent and subsequently recovering gallium alkoxides by distillation.
In the invention the aforesaid process for purifying the gallium alkoxides is characterized in that the amount of the potassium alkoxides added ranges from 1 to 3 equivalent to the amount of the chlorine impurity.
In the invention the aforesaid process for purifying the gallium alkoxides is characterized in that the organic solvent used in the process is a compound selected from toluene, xylene, alkanes having 6 to 10 carbon atoms, and cycloalkanes having 6 to 10 carbon atoms.
As an example, Ga(OiPr)3 is described below. In a general process for producing Ga(OiPr)3, GaCl3 is allowed to react with NaOiPr equivalent or slightly excess equivalent to the chloride in benzene or toluene as a solvent at a reflux temperature. Subsequently, NaCl by-produced is separated by filtration, the solvent is distilled away, and finally Ga(OiPr)3 is recovered by distillation. The distillation conditions are 1 Torr and approximately 120xc2x0 C.
The inventors have carried out the analysis of Ga(OiPr)3 prepared by the aforesaid process and found that the isopropoxide contains approximately 1 ppm of sodium and as much as 0.6 to 1.2 weight percent of chlorine.
The synthesis reaction of Ga(OiPr)3 is shown by the following formulas.
GaCl3+NaOiPr=GaCl2(OiPr)+NaCl xe2x80x83xe2x80x83(1) 
GaCl2(OiPr)+NaOiPr=GaCl(OiPr)2+NaCl xe2x80x83xe2x80x83(2) 
GaCl(OiPr)2+NaOiPr=Ga(OiPr)3+NaCl xe2x80x83xe2x80x83(3) 
In case where the reaction completely proceeds according to the formulas, the reaction of CaCl3 with NaOiPr equivalent thereto is to produce Ga(OiPr)3 without leaving GaCl(OiPr)2. However, in spite of the presence of excessive NaOiPr, the reaction of formula (3) practically seems difficult to finish, and it is presumed that a small amount of GaCl(OiPr)2 remains and contaminates Ga(OiPr)3 of the desired product. The removal of GaCl(OiPr)2 by distillation is difficult. It is known that Ga(OiPr)3 exists as a dimer [Ga(OiPr)3]2. On the other hand, it is presumed that GaCl(OiPr)2 exists as a complex Ga(OiPr)3xe2x80x94GaCl(OiPr)2, and close boiling points of the dimer and the complex make it difficult to separate GaCl(OiPr)2 by distillation.
The inventors have found that potassium isopropoxide (hereinafter represented by KOiPr) is added to Ga(OiPr)3 containing a small amount of GaCl(OiPr)2 to allow the reaction between both isopropoxides, subsequently Ga(OiPr)3 is recovered by distillation, and thereby the chlorine amount in the Ga(OiPr)3 can be decreased to approximately 0.02 percent with ease. Use of NaOiPr completely fails to exert this effect of KOiPr. Use of LiOiPr causes the gelation of the greater part of the reaction product to make difficult the recovery of the product by distillation. Of the alkali metal alkoxides, this major effect of KOiPr has not been forecasted at all.
The gallium alkoxides in the invention are compounds recovered by sublimation or distillation. Examples of such compounds include gallium isopropoxide, gallium methoxide, gallium ethoxide, gallium n-propoxide, gallium n-butoxide, gallium s-butoxide, and gallium t-butoxide. Of these, gallium isopropoxide has a high vapor pressure, can be distilled with ease, and used as a raw material for other gallium alkoxides. The gallium isopropoxide is, therefore, the most important as a target of the invention.
A potassium alkoxide used in the invention preferably is the same alkoxide as a gallium alkoxide to be purified. For example, for the purification of gallium isopropoxide, potassium isopropoxide is used. This can protect the gallium isopropoxide from contamination with another gallium alkoxide.
The amount of a potassium alkoxide used in the invention preferably ranges from 1 to 3 equivalents to the amount of the impurity chlorine. Although the amount of the potassium alkoxide is theoretically 1 equivalent thereto, the amount most preferably ranges from 1.1 to 1.5 equivalents in view of the reaction rate or the analytical error of chlorine. The addition of 3 equivalents or more of the potassium alkoxide fails to further reduce the amount of the impurity chlorine in a gallium alkoxide recovered by sublimation or distillation, and unpreferably leads to a serious decrease in the yield of the gallium alkoxide. The cause of the decrease in the yield may be attributed to the formation of a complex from the gallium alkoxide and the excess potassium alkoxide to cause great decrease in vapor pressure. Since the potassium alkoxide forms such complex, the gallium alkoxide recovered is not contaminated by impurity potassium.
The organic solvents used in the invention need to be those inactive to the gallium alkoxides, and preferred solvents are selected from toluene, xylene, alkanes having 6 to 10 carbon atoms, and cycloalkanes having 6 to 10 carbon atoms. The alkanes having 6 to 10 carbon atoms include hexane, heptane, octane, and decane. The cycloalkanes having 6 to 10 carbon atoms include cyclohexane.
The gallium alkoxides dissolve in these hydrocarbons such as toluene and octane more than in alcohols themselves forming the gallium alkoxides. Although the potassium alkoxides only slightly dissolve in the hydrocarbons such as toluene and octane, the alkoxides seem to dissolve well therein in the co-presence of the gallium alkoxides so that the reaction with the chlorine compounds proceeds with ease. Use of the hydrocarbon solvents makes it easiest to distill away the solvents from reaction solutions and preferably promises high yields of the gallium alkoxides by simple distillation.
On the other hand, use of the alcohols themselves forming the gallium alkoxides as the solvents for the reaction seems to cause slight gelation of the reaction solution, and slightly decreases the yields of the gallium alkoxides by simple distillation, although the effect of decreasing the chlorine content is maintained.
The reaction of the invention is carried out in an organic solvent with stirring at a reflux temperature of 70 to 140xc2x0 C. for 1 to 8 hours. After the reaction is complete, the solvent was distilled away at atmospheric pressure and then under reduced pressure, and subsequently a gallium alkoxide is recovered under reduced pressure by distillation or sublimation. The gallium alkoxides have relatively high heat stability to facilitate the recovery thereof.
The gallium alkoxides useful as the raw materials of the formation of dielectrics containing gallium oxide and as the raw materials of the syntheses of compounds for photoelectronics can be decreased in the amount of the impurity chlorine contained therein from approximately 1% to approximately 0.02% by the process of the invention.